This is pretty indispensible addon for me. It provides features that should be in Mozilla/Firefox and the author says it as such. What it gives you is a context menu to launch user definable programs for specific actions.

For example:

Assign mailto: for Firefox to launch new mail window for evolution, add this line for Mailer:

evolution-1.5 %r

Updated: David Woodhouse, has pointed out that mangling the url, is not a good idea, and one should just pass the mailto: url as is for evolution as shown above

To edit a text area in browser, like say Yahoo mail with vim, add this in your mozex preference for Text Area: